AU654759B2

AU654759B2 - Process of continuously casting metals

Info

Publication number: AU654759B2
Application number: AU32882/93A
Authority: AU
Inventors: Horst-Dieter Lindlar; Roland Schmoll; Wolfgang Schneider
Original assignee: Vereinigte Aluminium Werke AG; Vaw Aluminium AG
Current assignee: Maerz Gautschi Industrieofenanlagen GmbH Germany
Priority date: 1992-02-06
Filing date: 1993-02-05
Publication date: 1994-11-17
Anticipated expiration: 2013-02-05
Also published as: EP0560024B1; DE59302083D1; DE4203337C2; CA2088882C; US5343933A; NO180155C; JPH0691351A; DE4203337A1; NO930405D0; JPH0688105B2; ATE136239T1; AU3288293A; NO930405L; EP0560024A2; NO180155B; EP0560024A3; CA2088882A1

Abstract

In a method for the continuous casting of metals, especially aluminium or aluminium alloys, in a multi-mould casting installation, disturbances in the progress of casting are to be compensated for immediately so that optimum bar quality is obtained. This is achieved, according to the invention, by feeding in a pressurised gas and a lubricant underneath the hot top of each mould via at least one gas line, a regulating valve for setting the volume flow of gas, a downstream pressure sensor and a device for measuring the said volume flow being arranged in each gas line. It is furthermore envisaged, according to the invention, that the volume flow of gas in each gas line be automatically held constant at a predetermined value, irrespective of the respective filling level of the mould, in a first casting phase from the beginning of the filling of the mould with molten metal to a point in time following the entry of the metal strand into the water-cooled zone. During a second casting phase, the volume flow of gas in each gas line is automatically regulated in such a way that the gas pressure in each gas line is held constant at a predetermined value.

Description

l"UM/1 1 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990 4 b7 5 9

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodiged: Invention Title: PROCESS OF CONINUOUSLY CASING METALS The following statement is a full description of this invention, including the best method of performing it known to :-US Description Process of continuously casting metals The present invention relates to a process of continuously casting metals, especially aluminium or aluminium alloys, in a multiple mould casting system, with each mould being provided with a hot top attachment and with a pressurised gas and a lubricant being introduced into the mould cavity underneath the hot top attachment.

A process of this type is described in EP 0 218 855 for example. In this case, the continuous casting mould is provided with a hot top attachment whose inner wall forms an e. overhang and protudes beyond the inner wall of the continuous casting mould. It is at this overhang where the pressurised gas together with the lubricant is introduced into the cavity of the continous casting mould. During the S. entire casting phase, the gas is introduced at a constant flow rate. In the case of multiple mould casting systems the gas supply system is designed in such a way that all moulds are supplied with the same constant quantity of gas.

However, it has been found that with this type of operation, good results in respect of surface quality and quality of edge structure of all continuously cast billets can be achieved only if casting conditions are completely interference-free. But in practice, such conditions hardly ever exist. Especially in multiple mould casting systems again and again it is found that certain moulds require different quantities of gas. Furthermore, the gas requirements of individual moulds may change during the casting operation. In particular, this applies to moulds with a diameter in excess of 25 cm. In addition, it has been found that the setting for the gas quantity has to be -2monitored regularly. Even under normal casting conditions it cannot be excluded that the quantity of gas required by one single mould changes. 'in consequence, with this type of operation it is not possible to achieve uniformly good billet qualities because again and again it is possible, within one mould system, to find billets whose overall quality is reduced and/or whose quality changes considerably along the length of the casting.

EP 0 449 771 proposes a further process of the initially mentioned type in the case of which a higher quantity of gas is set for the start of the filling operation, which gas quantity is reduced considerably as the level of metal in the mould rises. When the billet subsequently enters the water-cooled zone, a cold run occurs due to a 'higher degree of shrinkage of the billet. The gap between the metal and mould wall increases in the process so that a very large quantity of gas is required to maintain the pressure pad in the mould cavity. This process usually does not occur exactly simultaneously and to the same degree in the individual moulds of a multiple casting system so that, to *maintain the gas pad, the moulds require different quantities of gas. This also applies to other types of interference which may occur in the individual moulds during the casting process, such as the occurrence of a crack in the hot .top or insufficient lubrication of the. inner mould wall due to interference in the supply of separating agents.

According to the process described, the gas supply can be controlled simultaneously only (to the same degree) for all moulds within the main gas supply line. In this way it is not possible to ensure that the necessary gas pad is maintained in each individual mould. This necessarily leads to at least some billets produced in one casting process suffering from a: reduction in quality.

it is therefore the object of the present invention to develop a process in the case of which any interlerence in the casting operation is compensated for directly so that an optimum billet quality is achieved. in particular, it is 3 the object to obtain billets with a high surface quality and a high quality of edge structure in multiple mould casting systems.

In accordance with the invention, the objective is achieved by a process having the characteristics as given in claim 1. Advantageous embodiments of the process in accordance with the invention result from the sub-claims.

In accordance with the process as proposed by the invention, the gas for each mould of a multiple mould casting system is supplied via at least one gas pipeline. Each gas pipeline comprises a control valve for setting the flow of gas volume, a subsequently connected pressure sensor and a device for recording the flow of gas volume. During a first casting phase extending from the moment when the mould starts to be filled with liquid metal up to the point in time after the metal billet has entered the water-cooled region the flow of gas volume is automatically kept constant at a predetermined value, independently of the respective filling level of the mould. During the subsequent second casting phase, the flow of gas volume in each gas pipeline is automatically controlled in such a way that the gas pressure in the pipeline is kept constant at a predetermined value.

In this way it is possible to prevent or quickly stop any cold run problems during the initial casting phase and any interference in the casting operation during the stationary casting phase.

The principle of the gas supply system as used for the process in accordance with the invention is diagrammatically illustrated in Fig. 1. The gas pipelines 2 branch off from the main gas pipeline 1 and lead to the individual moulds of the multiple mould casting system, with at least one gas pipeline 2 leading to each mould. Each gas pipeline 2 comprises a measuring and control unit 3 for measuring and controlling the flow of gas volume and the gas pressure.

Fig. 2 diagrammatically illustrates the operating principles of the measuring and control units. The gas pipeline 2 contains a device 4 which comprises a measuring instrument for recording the flow of gas volume, and an electronically controllable control valve for setting the flow of gas volume. A pressure sensor 5 measures the actual value of the gas pressure in the gas pipeline 2. A nominal pressure value, optionally also including an upper and/or lower limit value for the flow of gas volume, or, alternatively, a nominal value for the flow of gas volume may be predetermined in an electronic control unit 6.

S* The control valve is controlled by the control unit 6 in accordance with the predetermined values. The values to be set may optionally be fed in by a S process computer 7, e.g. in accordance with pre-selectable casting programs for different types of moulds and/or different alloys. The inner diameter of gas 1 pipeline 2 is at least 6 mm.

In a preferred embodiment of the process in accordance with the invention, a nominal pressure value for the individual gas pipelines leading to the moulds is predetermined. In such a case, the flow of gas volume in each gas pipeline from the onset of casting (empty moulds) is controlled in such a way that 20 the flow of gas volume is increased if the pressure measured in the gas pipeline is lower than the nominal pressure value, and it is decreased if the pressure measured is higher than the nominal pressure value. The flow of gas volume is limited to a predetermined maximum value because otherwise, if there was no counter-pressure, an unlimited amount of air would enter. At the same time, this type of process ensures that the flow of gas volume during the initial casting phase remains constant at a predetermined maximum value until the mould has cooled to such an extent that the metallostatic pressure in the mould corresponds to the predetermined nominal pressure value. In accordance with the process as proposed by the invention, 5 the operation of filling the moulds is controlled in such a way that this point is not reached until the cast billets have reached the water-cooled region.

Figure 3 illustrates the casting sequence of such a process, using the time-dependent values for the metal level in the mould and for the flow of gas volume as well as the gas pressure in the gas pipeline leading to a mould. The process of filling the mould begins at the point in time tA0. From the onset of the filling operation, the flow of gas volume has the predetermined maximum value. The pressure measured in the gas pipeline rises as the metal level in the mould rises. When the metal has reached a level which is preferably 50% to 85% below the maximum filling level, the level of metal in the mould is initially kept constant at such a value (point in time tAl). The gas pressure remains constant accordingly. At approximately this point in time, the casting table is lowered. At the point in time tA2 the lower part of the cast billet inters the water-cooled region (direct cooling). Until a cast length approximately corresponding to half the billet diameter or half the billet thickness is reached (tA3), the level of metal in the mould is kept constant, with a uniform maximum flow of gas volume.

In this way it is ensured that in spite of an increasing gap between the metal and mould wall due to a higher degree of shrinkage of the billet, an adequate gas pad is maintained in this critical region.

Subsequently, the level of metal is made to rise further.

The gas pressure increases accordingly, and the flow of gas volume remains constant until the measured gas pressure has reached the predetermined nominal pressure value. In the example given, this is the case at the point in time tA4.

In accordance with pressure losses possibly occurring in the gas pipeline at a maximum flow of gas volume (depending on the cross-section and length of the individual gas pipelines), this point in time is reached shortly before the mould is filled completely. From this point in time onwards, -6the gas pressure is automatically kept constant at the predetermined nominal pressure value. The flow of gas volume required for madintaining this pressure clearly drops up to the point in time (tA5) when the mould is filled completely.

During the further casting sequences, under normal operating conditions, only slight changes in the flow of gas volume are required for accurately keeping the pressure constant at the predetermined nominal value. Emptying of the mould starts at the point in time tA6V As the level of metal is lowered, the flow of gas volume increases to the predetermined maximum value if the gas pressure continues to be kept constant. After the point in time t A71 the gas pressure decreases to zero, with the mould being completely :empty.

The above-described pressure control system may also be used for a continuously rising mould filling level. The filling *speed is then controlled in such a way that the level of metal at which the measured pressure in the gas pipeline corresponds to the predetermined nominal value is not reached until after the cast billets have entered the direct cooling region.

According to a further embodiment of the process in accordance with the invention, it is also possible to operate at higher filling speeds. In such a case, a nominal value for the flow of gas volume is predetermined in the first casting phase. Independently of the gas pressure, the flow of gas volume is kept constant at this value until after the cast billets have entered the direct cooling region. only then is it permitted to switch over to a constant pressure control. A casting sequence possible in accordaiice with this embodiment is illustrated in Fig. 4.

The process of filling the moulds begins at the point in t ime t BO' From the start of the filling operation, the flow of gas volume is kept constant at the predetermined nominal value. This nominal value is preferably selected in accordance with the maximum value of the flow of gas volume at a constant pressure control. Lowering of the castiitable -7 commences at the point ini time tBl' The pressure measured in the gas pipeline increases with a rising level of metal and reaches a. maximum value at tB2' with the mould being filled completely. This maximum value is in excess of the nominal pressure value predetermined for the second casting phase.

This is due to the pressure losses possibly occurring in the gas pipeline at a maximum flow of gas volume (depending on the cross-section and length of the individual gas pipelines). At the point in time t B31 the cast billets enter the direct cooling zone. The flow of gas volume is kept constant at the' predetermined nominal value up to the point in time t B*This means that in this application, too, a sufficient gas pad is ensured in the critical phase when the billet enters the direct cooling zone. It is only at this point in time that the change-over to constant pressure control in accordance with the description of Figure 3 takes a:place. The gas pressure is reduced to the predetermined nominal pressure value and during the further casting operation is kept constant at this value. If a maximum value for the flow of gas volume is predetermined for the phase of constant pressure control, emptying of the moulds takes place as described in connection with Figure 3.

The maximum or nominal value to be determined for the flow of gas volume in accordance with the process proposed by the invention is independent of the level of metal in the -mould. It is determined as a function of the shape of billet to be cast. in the case of continuously casting aluminium.

and /its alloys, the vOaues to be used range between 0.2 and Nl/h per mm circumference of the cavity of the respective mould. To achieve optimum casting conditions, a value of approx. 0.32 Nl/h per mm circumference of the cavity of the mould used has been found to be particularly advantageous.

By specifying such a maximum value for the f low of gas volume it is possible not only to achieve the advantages mentioned above but also to ensure that, if unusual defects occur such as the formation of cracks or leaks in the gas supply system, an unlimited high flow of gas volume cannot be set.

-8ain a further preferred embodiment of the process in accordance 'With the invention, the range of the flow of gas volume is set a lower limit by predetermining a minimum value. In this way it is ensured that even if there is interference in the casting sequence, which leads to a high counter pressure which is in excess of the predetermined nominal pressure value or the metallostatic pressure of the melt, for instance if the passage of gas in the casting direction is obstructed, a minimum flow of gas volume is introduced into the mould cavity to that a gas pad between the metal and mould, wall is maintained. For aluminium and aluminium alloys, values ranging between 10 and 130 Nl/h which are independent of the mould cavity have been found to be advantageous. Preferably, a minimum value of approx. Nl/h is predetermined.

4 in the case of the method of operation according to Figures 3 and 4, the f low of gas volume at the end of the casting phase has the set maximum value. As the level of metal in the mould is lowered, it is not possible to prevent gas from being blown through the melt. This may lead to a deterioration of the billet quality in the region of the top .00. end, for example as a result of oxide inclusions and/or undesirably high gas contents. in such a case, more metal has to be cut of f at the top end of the billet, which leads to considerable metal losses. This may be avoided, for example, by reducing, in stages or continuously, the predetermined nominal pressure value after a certain cast billet length or casting time has been arrived at, as la result of which the flow of gas volume is automatically lowered when emptying the mould. A further possibility consists in predetermining a constantly low flow of gas volume during this end phase. The values to be set in such a case are preferably selected from the above-mentioned range of minimum. values for the flow of gas volume. The reduced values for the nominal pressure value for the flow of gas 9 volume are preferably predetermined by a program of the process computer 7 (Fig. 2).

To ensure accurate control of the gas supply, the pre-.pressure of the gas in the main gas pipeline is set to a value of at least 2 bar. The minimum inner diameter of the gas pipelines leading to the individual moulds is selected to be such that the pressure losses in the gas pipelines at the gas flow values occurring in the gas pipelines during the second casting phase (constant pressure control) are negigibly low. Under such conditions, the nominal pressure value can be set to be such that, when the mould is filled almost completely, it is almost identical to the metallostatic pressure or is only slightly in excess thereof. In particular, these conditions are reached if the inner diameter of the gas pipelines amounts to at least 6 The process in accordance with the invention can advantageously he used for continuously casting aluminium *and al~uminium, alloys in round billet moulds (circular *cross-section) rolling billet moulds (rectangular cross-section) and oval billet moulds -with straight side walls and semi-circular end walls. As in accordance with the process proposed by the invention, the air supply to the individual moul~ds is controlled separately, it is possible, especially when casting rolling billets, to use moulds of se. different types and/or dimensions in the same multiple mould casting system. In such a case, the process parameters to be predetermined are adapted to the respective mould types.

In the case of large mould types, especially with rolling or oval billet moulds with cross-sections from approx. 1050 300 mm it has been found to be advantageous to supply the gas to the individual moulds via several gas sub-pipelines, with, for example., 1 to 2 gas sub-pipelines being guided to each mould side and I gas sub-pipeline to each mould end.

The flow of gas volume and pressure are measured and controlled separately in each gas sub-pipeline in accordance with Figure 2, the flow of gas volume in each gas sub-pipeline being allocated an upper limit in the' form of a percentage of the total maxcimum value predetermined for the respective mould, such percentage being dependent on the distance between the gas sub-pipelines on the circumference of the mould cavity. The nominal' pressure value to be predetermined for each gas sub-pipeline is unaffected by the number of gas sub-pipelines per mould.

Air or nitrogen are particularly suitable gases for the process in accordance with the invention.

$see #0 06 0066 6* 6J A substantial advantage of the process in accordance with the invention, inter alia, consists in that the lubricant supplied together with the gas can be introduced with a constant flow of volume, which means that, as far as the lubricant supply is concerned, there is no need for a great deal of control facilities. To maintain optimum casting conditions, the lubricant is introduced at a constant flow of. volume ranging between 0.1 and 1.0 ml/h per mm circumference of the cavity of the respective mould. it is advantageous to use lubricants whose viscosity at 40 0

C

ranges between 35 and 220pp 2 Is. In particular, this group includes beet oil and castor oil.

The process in accordance with the invention is used for continuously casting simultaneously in multiple mould casting systems in the case of which, in the stationary casting phase, operations take place at a constantly high level of metal in the moulds. The individual moulds are filled simultaneously. Equally, the cast billets are lowered simultaneously via a casting table. Under the conditions as described, it is possible, even in the initial casting phase, to build up an adequate gas pad in each mould of the system and maintain it during the entire casting phase. As the gas supply is controlled separately for each mould, each mould receives the exact amount of air which ensures optimum 11 operating conditions. In this way it is possible in such a system to produce largely defect-free billets with a constantly high surface quality. Any cold run problems are avoided when the billets enter the direct cooling zone. Any interference which might occur in the stationary casting phase is compensated for directly or avoided altogether due to the fact that the gas pressure can be kept accurately constant through automatic control of the flow of gas volume, even if slight deviations from the predetermined nominal value occur. Furthermore, by specifying suitable casting programs via a process computer it is possible to build up an almost fully automatic casting system.

0* o e 5*55 *e

Claims

1. A process of continuously casting metals, especially aluminium or aluminium alloys, in a multiple mould casting system, with each mould being provided with a hot top attachment and with a pressurised gas and a lubricant being introduced into the mould cavity underneath the hot top attachment, characterised in that the gas of each mould is guided through at least one gas pipeline, with a control valve for setting the flow of the gas volume and a subsequently conPnated pressure sensor S" as well as a device for recording the flow of the gas volume being arranged in each gas pipeline, that in a first casting phase from the moment of filling the mould with liquid metal up to the point in time after the metal billet has entered Sthe water-cooled region, the flow of gas volume in each gas pipeline is automatically kept constant at a predetermined value, independently of the respective filling level of the mould, and that in a second casting phase, the flow of gas volume in each gas pipeline is automatically controlled in such a way that the gas pressure in each gas pipeline is kept constant at a predetermined value.

2. A process according to claim 1, characterised in that the operation of filling the mould with liquid metal takes place in such a way that the mould is filled completely before the billet enters the water-cooled region, with the flow of gas volume in each gas pipeline, independently of the gas pressure in the gas pipeline, being kept constant at the predetermined value up to a point in time after the metal billet has entered the water-cooled region and that after such point in time, the flow of gas volume in each gas pipeline is automatically controlled in such a way that the gas pressure in each gas pipeline is kept constant at the predetermined value. 13

3. A process according to claim 2, characterised in that during the second casting phase, the actual value of the gas pressure in each gas pipeline is measured and compared with a predetermined nominal value and that the f low of gas volume is increased if the actual value of the gas pressure is lower than the predetermined nominal value and decreased if the actual value of the gas pressure is higher than the predetermined nominal value.

4. A process according to claim 1, characterised in that the operation of filling the moulds with liquid metal takes place in such a way that the metal billet enters the water-cooled region prior to the mould being filled completely, with the flow of gas volume in each gas pipeline being kept constant up to the point in time which exists between the metal billet entering the water-cooled region and the mould being filled completely and at which the gas pressure in the gas pipeline reaches the predetermined value *Soo and that after such point in time, the flow of gas volume in each gas pipeline is automatically controlled in such a way o that the gas pressure in each pipeline is kept constant at the predetermined value. A process according to claim 4, characterised in that within the initial casting phase up to a point in time after the metal billet has entered the water-cooled region, the metal bath level is kept constant at a low value which is between 50% and 85% below the maximum filling level in the hot top and that thereafter the mould is filled completely.

6. A process according to any one of claims 4 or characterised in 14 that within the first and the second casting phase the actual value of the gas pressure in each gas pipeline is 4 measured and compared with a predetermined nominal value and that the flow of gas volume is increased if the actual value of the gas pressure is lower than the predetermined nominal value and decreased if the actual value of the gas pressure is higher than the predetermined nominal value.

7. A process according to any one of the preceding claims, characterised in that the flow of gas volume is limited by a predetermined maximum value. S. A process according to claim 7, characterised in that the predetermined maximum value is a value between 0.2 and 2.0 Nl/h per nma circumference of the mould cavity.

9. A process 4ccording to claim 8, characterised in that the predetermined maximum value is a value of approx. 0.32 Nil/h per mm circumference of the mould cavity. A process according to any one of the preceding claims, char'acterised in. that the flow of gas volume is limited by a predetermined minimum value.

11. A process according to claim characterised in that, independently of the circumference of the mould cavity, the predetermined minimum value is a value between and 130 Ni/h.

12. A process according to claim 11, characterised in that the predetermined minimum value is approximately Nl/h. 15

13. A process according to any one of the preceding claims, characterised in that tBe predetermined nominal value for the gas pressure in each gas pipeline corresponds to at least the metallostatic pressure of the melt when the mould is filled completely.

14. A process according to any of the preceding claims, characterised in that after a predetermined casting length or casting time has been reached, the predetermined nominal value for the gas pressure is decreased in stages or continuously.

15. A process according to any one of claims 1 to 13, 6characterised in that after a predetermined casting length or casting time has been reached, the flow of gas volume is reduced to a predetermined constant value. 1. A process according to claim characterised in that after a predetermined castin Length or casting time has been reached, the flow of gas volume is kept constant at the predetermined minimum value.

17. A process according to any one of the preceding claims, characterised in that ekoh pre-pressure of the gas existing in front of the individual control valves for setting the flow of gas volume is set to a value of at least 2 bar.

18. A process according to any one of the preceding claims, characterised in that the minimum inner diameter of the gas pipelines is selected to be such that with a controlled flow of gas volume the pressure' losses in the gas pipelines are F negiglibly small as compared to the predetermined nominal value for the gas pressure. 16

19. A. process according to any one of the preceding claims, characterised in that the inner diameter of the gas pipelines amounts to at least 6 mm. A process according to any one of the preceding claims, characterised in that each mould is supplied with gas via a plurality of gas sub-pipelines, with the flow of gas volume and gas pressure of each gas sub-pipeline being measured and controlled separately.- :e 21. A process according to claim characterised in that the flow ol: gas volume for each gas sub-pipeline of a mould has an upper limit consisting in part of the maximum value predetermined for the mould.

22. A process according to any one of claims 20 or 21, characterised in that the same nominal gas pressure value is predetermined for each gas sub-pipeline of a mo-.1ld, said nominal value corresponding to at least the metallostatic pressure of the melt when the mould is filled completely. too. 23. A process according to any one of the preceding claims, tool characterised in that the gas used is air or nitrogen.

24. A process according to any one of the preceding claims, characterised in that the lubticant is introduced at a constant flow of volume. A process according to any one of the preceding claims, characterised in that the lubricant is introduced at a flow of volume ranging between 0.1 and 1.0 mi/h per mm circumference of the mould cavity. -17

26. A process according to any oxie of the preceding claims, characterised in -i that the kinematic viscosity, at 40 OC ranges between 35 and 220 wM 2 /S.N

27. A process according to claim 26, characterised in that the lubricant is beet oil or castor oil.

28. A process according to any one of the preceding c'Laims, characterised in that the mnoulds used are round billet moulds with a circular cross-section. e~e~c. 29. A proces according to any one of the preceding claims, characterised in that the moulds used are rolling ingot moulds with a rectangular cross-section.

30. A process according to any one of the preceding claims, 6O characterised in that the moulds used are oval ingot moulds with straight side walls and semi-circular end walls. e e

31. A process according to any one of the preceding claims, CS characterised in that if moulds with different dimensions are used in the same multiple casting system, different nominal gas pressure values, are predetermined. DATED this 4th day of Februiary 1993. VAW ALUMINIUM AG WATERMARK PATENT TRADEMARK ATT'ORNEYS "THE ATRIUM" 290 BURWCX)D ROAD HAWTHORN. VIC. 3122 IV ABSTRACT A process of continuously casting metals, especially aluminium or aluminium alloys, in a multiple mould casting system is disclosed with each mould being provided with a hot top attachment and with a pressurised gas and a lubricant being introduced into the mould cavity underneath the hot top attachment, characterised in that the gas of each mould is guided through at least one gas pipeline, with a control valve for setting the flow of the gas volume and a subsequently connected pressure sensor as well as a device for recording the flow of the gas volume being arranged in each gas pipeline, that in a first casting phase from the moment of filling the mould with liquid metal up to the point in time after the metal billet has entered the water-cooled region, the flow of gas volume in each gas pipeline is automatically kept constant at a predetermined 9 9 S" value, independently of the respective filling level of the mould, and that in a second casting phase, the flow of gas volume in each gas pipeline is automatically controlled in such a way that the gas pressure in each gas pipeline is kept constant at a predetermined value. 99 9 .j 99 o 99 O* S